The present invention relates to a plasma display panel having a protective film for reducing variation in a discharge delay time within a substrate surface.
In recent years, flat display panels, such as liquid crystal display panels (LCDs), field emission display panels (FEDs), and plasma display panels (hereinafter referred to as PDPs) have been drawing attention as display devices of which the size can be increased and the thickness reduced from among color display devices used for displaying images, such as computers and televisions. From among these, PDPs have particularly excellent characteristics in terms of high speed response, wide view angle, and the like, and have been actively developed in order to increase the definition and quality of images.
PDPs are basically formed of a front plate and a rear plate. The front plate is formed of a glass substrate, display electrodes formed of scanning electrodes made of transparent electrodes in stripe form and sustain electrodes made of bus electrodes formed on one main surface, as well as support electrodes, a dielectric layer which covers the display electrodes so as to function as a capacitor and forms a wall charge through discharge, and a protective film formed on the dielectric layer. Meanwhile, the rear plate is formed of a glass substrate, address electrodes in stripe form formed on one main surface in a direction crossing the display electrodes, a dielectric layer covering the address electrodes, partitions formed on the dielectric layer, and substance layers formed between the partitions, each of which emits red, green, or blue light.
The front plate and the rear plate are air-tight-sealed together so that the sides on which electrodes are formed face each other, and spaces between these are sealed air tight, and a discharge gas, such as Ne—Xe, is sealed in the discharge spaces where discharge cells are formed on the partitions under pressure of 400 Torr to 600 Torr. A video signal voltage is selectively applied to the display electrodes, and thus, a discharge gas is discharged, which then generates ultraviolet rays so that the substance layers of each color are excited and emit red, green, and blue light, and thus display a color image.
Metal oxide films, such as of magnesium oxide (MgO), having excellent resistance to sputtering and excellent secondary electron discharging properties are formed in a thin film process, for example an electron beam vapor deposition method, as a protective film formed on the dielectric layer of the front plate, and are widely used. The excellent resistance to sputtering of MgO allows the dielectric layer to be protected from ion impact (sputtering) resulting from discharge. In addition, secondary electrons are efficiently discharged into the discharge cells as a result of the excellent secondary electron discharging properties, and thus, the protective film has a function of lowering the voltage at which discharge starts.
In addition, it is known that the film quality and properties of MgO thin films used as protective films vary, due to differences resulting from oxygen deficiency and mixing in of impurities. In the process for forming a protective film, an oxygen (O2) gas is supplied into, for example, an electron beam vapor deposition chamber, under a predetermined partial pressure, and thus, an amount for oxygen deficiency in the MgO thin film is adjusted so that the film is formed with target properties under control. There is easily an oxygen deficiency in films formed without supply of O2 gas, because oxygen atoms easily come off from the film material when the metal oxide, for example MgO, which is the film material, vaporizes as a result of irradiation with an electron beam. Accordingly, it is necessary to supply an O2 gas to the growing surface all of the time.
Demand for increase in the size of screens and definition has been increasing for PDPs, due to large screen size full high vision screens, and thus, it is more desired for the area of screens and the number of scanning lines to be increased, and at the same time, for the corresponding addressing period to be shortened. In order to shorten the addressing period, the protective film is required to have higher secondary electron discharging performance. That is to say, it is necessary to increase the number of scanning lines as the discharge cell structure becomes highly finer, and for the pulse width of address pulses applied during the addressing period to be narrower, so that drive can be carried out at higher speed. In terms of the discharge phenomenon, there is a discharge delay, such that actual discharge occurs considerably later than the rise in the applied pulse. Therefore, the probability of discharge being completed within the applied pulse width becomes low, causing failure in light-up, so that write-in cannot be carried out in cells which should be lit up, and thus, flickering can be observed on the screen display. In order to shorten this discharge delay time, higher secondary electron discharging performance is required for the protective film.
Examples have been disclosed, where the index of refraction of a protective film is adjusted to 1.4 to 2.0 for light having a wavelength of 400 nm to 1000 nm, and thus, the discharge delay time shortens and the resistance to sputtering increases, so that excellent display quality can be maintained (see for example Japanese Unexamined Patent Publication No. 2003-317631).
In addition, methods have been disclosed, according to which a hydrogen plasma process is carried out on a protective film that is formed by introducing an O2 gas, and thus, the volume resistivity of the protective film is adjusted to 3.5×1011 Ω·cm or higher, or three or more hydrogen atoms are contained per 100 atoms in the entirety of the protective film at that time, and thus, shortening of the discharge delay time and lowering of the discharge voltage can be achieved (see for example Japanese Unexamined Patent Publication No. 2002-33053).
It is known that the discharge delay time varies depending on the film thickness of the MgO thin film. This is considered to be because the degree of crystal growth in the MgO thin film is different depending on the film thickness, which causes a difference in the secondary electron discharging properties. Accordingly, the distribution in the film thickness of the protective film affects the distribution in the discharge delay time within the substrate surface. On high definition screens of conventional PDP where the resolution is 1366×768, images can be displayed without such defects as failure of light-up occurring in the case where the distribution of the discharge delay time within the substrate surface is within ±50%. On full high definition screens where the resolution of the PDP is 1920×1080, however, there are issues causing defects in image display unless the distribution in the discharge delay time within the substrate surface is further reduced.
Accordingly, further reduction in the discharge delay time and uniformity on the surface has been required for higher definition PDPs having higher image quality in recent years. In the case where a protective film according to the technology in the above described Japanese Unexamined Patent Publication No. 2003-317631 or Japanese Unexamined Patent Publication No. 2002-33053 is used, an unstable PDP in which the distribution in the discharge delay time within the substrate surface is in a range of ±40% or greater resulting in non-uniformity. Therefore, there is an issue, such that there are discharge cells which do not have sufficient discharge properties on the surface, causing defects in light-up or flickering on the display, including failure in light-up or errors in discharge during initialization.
The present invention is provided in order to solve the above described issues, and an object thereof is to provide PDP (plasma display panel) having a protective film for reducing variation in a discharge delay time within a substrate surface.